1. Field of the Invention
This invention relates to alloy powders, in particular, to compositions of such powders useful for forming high hardness metal parts by powder metallurgy (P/M), and to processes for making and using such compositions.
2. Brief Description of the Background Art
Powder metallurgy is a process of imparting high pressure to highly purified, substantially uniform ferrous powders to produce ferrous parts with high densities. The process is also known as "pressure forging." Sinterhardening is a P/M process in which P/M parts transform partially or completely into martensite during the cooling phase of a sintering cycle.
In both P/M and sinterhardening, minor amounts of secondary metals are typically added to the base P/M material to improve its hardenability. In order to achieve optimum hardenability, prealloying techniques are generally preferable to elemental additions.
Manganese is added to typical commercial steels in the range of 0.25 to 1.0% to increase strength and hardenability of plain carbon steels. Chromium is also commonly added to improve hardenability, strength and wear resistance of conventional steels. However, in steel powders for use in powder metallurgy, e.g., powders having an average particle size of from 55 to 100 microns, manganese and chromium contents are generally maintained below 0.3% in order to reduce oxide formation during annealing, "Design Criteria for the Manufacturing of Low Alloy Steel Powders", Advances in Powder Metallurgy, vol. 5, 1991, pp. 45-58.
Molybdenum and nickel are commonly used in low alloy P/M steel powders because their oxides are easily reduced during the annealing treatment of the water-atomized powders. Molybdenum and nickel efficiently increase the strength and the hardenability of steels, while nickel also increases the strength, toughness and fatigue resistance of the steel, S. H. Avner, Introduction to Physical Metallurgy, McGraw-Hill, N.Y., 1974, pp. 349-361. These elements are however more expensive than manganese and chromium and are subject to large price variations which have an obvious deleterious effect on the steel powder price.
Sinterhardening is an attractive technique for the manufacturing of high hardness P/M parts because it eliminates the need for post-sintering heat treatment, thus significantly reducing processing costs. Furthermore, high thermal stresses and part distortion resulting from conventional quenching are avoided, providing improved control of final part dimensions. Previous techniques for producing low alloy steel powders for P/M application include acid treatment to remove the oxide layer in U.S. Pat. No. 3,764,295 to Hoganas and use of high carbon (0.1 to 0.70%) in the annealed powder in British Patent No. 1,564,737. In contrast, the present invention eliminates the acid treatment while maintaining oxygen and carbon at low concentrations in order to improve compressibility and minimize powder oxidation during the atomizing and annealing process. Because of these parameters, the present invention is capable of producing a steel powder with high hardenability and minimal oxygen content.